1. Field of the Invention
The invention relates to a magnetic separator formed of parallel discs extending radially from a horizontal shaft.
It is well-known that liquids contaminated with suspended particles or dissolved high-molecular substances, such as resinous substances, can be cleaned by adding a ferromagnetic particulate material, such as magnetite, iron, cobalt or nickel, to the liquid and separating the contamination together with the ferromagnetic material in a magnetic field. It is also known to clean liquids in a similar way, which liquids from the start contain contaminations of ferromagnetic particulate materials, for example departing coolant from machines for mechanical machining, such as lathes and drills. Also contaminated gases can be cleaned in principle by the addition of a ferromagnetic particulate material and treatment in a magnetic field for separation of the contaminations. It is also known to use chemical flocking agents simultaneously in the cleaning process, for example lime, alum, iron chloride, polyelectrolytes and water glass.
2. The Prior Art
In the described cleaning operations a magnetic separator is used. One known embodiment of such a separator consists of a rotatable cylindrical drum which is lowered down into a trough which is concentric with the drum, so that a gap is formed between the envelope surface of the drum and the trough. A plurality of permanent magnets are arranged in longitudinal rows on the inside of the envelope surface of the drum, and the medium to be cleaned is led through the gap at the same time as the drum is rotated. The separator is also provided with a scraper for removing the material which adheres to the drum during the separation so that the process is continuous. The magnets can either rotate with the drum or be stationary during the rotation of the drum.
Another known embodiment of a magnetic separator is formed with a gap between two plane parallel rigid walls, one of which contains a plurality of horseshoe magnets built into it, and located adjacent to and spaced from each other.
Proposals have also been made to provide magnetic separators, the walls of which consist of ferromagnetic material and are attached, radially directed, along a rotatable shaft, the separator being provided with a stationary magnet with the ability to generate a magnetic field, substantially parallel with the rotatable shaft, with local gradients. To achieve a practical embodiment of such a separator, bulky and very expensive magnetization devices are required, such as an iron circuit and a magnetization coil. The scraper means may be of a finger-like type.
According to the Lundquist application, Ser. No. 440,872, filed Feb. 8, 1974, there is provided a magnetic separator with an extremely large separating surface without the use of bulky and expensive magnetization devices and with a very efficient scraping during practical operation. In this way an extremely compact separator is achieved. This result is obtained by designing the separator as a disc filter, in the filter discs of which permanent magnets are arranged in such a way that local field inhomogeneities occur in the gaps between the discs, and by designing the scraper means as endless transport members which enter into the gaps and carry away the material removed from the filter discs.
The filter discs in the separator according to the Lundquist invention are normally constructed with smooth outer walls between which the permanent magnets are positioned. The walls then consist of a non-magnetic material, for example stainless steel-sheet, aluminium or resin, for example an epoxy resin into which the permanent magnets are then suitably cast.
According to one embodiment of Lundquist the permanent magnets are in the form of discrete magnets arranged to extend between supporting walls of the discs, said walls facing the gaps. The magnets should then have great coercive field strength in order that they may be made short and the separator thus compact. Particularly preferred are ceramic magnets such as barium or strontium ferrite, which have a coercive field strength exceeding 100 kA/m, but in principle it is also possible to use metallic magnets with great coercive field strength, such as samarium-cobalt magnets. It is particularly favorable to use anisotropic magnets, since these have greater coercive field strength than the corresponding isotropic magnets, for barium and strontium ferrite, for example, a coercive field strength of more than 200 kA/m.
In order to make the filter surface as large as possible, it is suitable to arrange the permanent magnets along substantially the whole extension of the discs. To facilitate the scraping off it may be advantageous, however, to omit magnets within limited, preferably sector-shaped areas, since agglomerated particulate material can be detached more easily from the discs if they are provided with distinct areas without magnets.